<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Inner</title>
</head>
<body>
  <ul>
    <li id="fir">一</li>
    <li id="sec"></li>
    <li id="thi">三</li>
    <li id="for">四</li>
  </ul>
  <script src="./inner.js"></script>
</body>
</html>